I just want to get a poll here. I have noticed that just about every factory turbo car (across manufacturers) has an OEM overboost fuel cut designed/intended to save the engine. Some say it will in fact damage/blow the engine, on the logic that cutting fuel and combustion completely is like any other lean condition. I'm not talking about aftermarket "soft cut" ignition limiters.

So who has firsthand experience with a blown motor directly resulting from overboost fuel cut? When I say "direct result," to make it simple I mean that you hit fuel cut and the car had a damaged apex/corner/side seal or a cracked plate almost immediately following the event.

 

I see frequent recommendations to add a FCD when a noob has just opened up the exhaust and is now experiencing fuel cut. They add the FCD. Then they experience boost creep.

The next poll question is "Who has added an FCD and then blown the engine before they got around to porting the wastegate?"

 

So, who voted yes? Someone did already.

 

Everyone who votes yes, please explain the situation including modifications, how you knew it was blown, and when you knew it was blown.

 

99% sure that I blew my first motor 'cause of it. The previous owner said he had done stuff to the car (Rtek, walbro, etc.) that he didn't. So I drove it like it did. Turns out he hadn't and I was frequently hitting the fuel cut. So it was definately a contributing factor.

 

agreed. I would assume a lot more people have blown engines because the FCD was installed. Im sure a decent amount of people have blown it from over boosting because all they did was change out the exhaust and nothing else than from fuel being cut from the engine.

 

RX-7 owner: 21 years non-stop
Least RX-7s owned: 1
Most RX-7s owned: 4
Current RX-7s owned: 2
Number of blown engines: 0

Keeping up with maintenance, not cutting corners, and having a lot of rotary mechanics and tuners as friends: Priceless

I ran my S4 TII with a straight-through catback, stock wastegate, and FCD for years with no problems.

 

Point taken. I probably should have mentioned lack of fuel mods in that question.

 

so far nobody has said that they hit fuel cut and immediately ran on one rotor

 

This didn't happen to me personally, but when I bought my 7 it had a blown motor (rear apex seal), rb turbo back exhaust, a fuel cut defencer, and an s4 turbo.

I dunno about you, but that pretty much screams overboost to me.

 

Who would've thought that a system designed to save engines usually saves engines?

 

Like he said, it is designed to prevent engine damage, so unless you are going strictly pre-mix with oil injection removed, it should do the job.

 

Cutting all fuel on overboost is actually quite safe. Since all fuel is cut, it is not like a lean condition at all, it's like a no fuel condition. So the engine just stops running at that point. Now if the system cut half the fuel, yeah, that would be a problem.

The only time damage is going to occur is if there is something else wrong with the car such as marginal injectors.

Having never owned a factory turbo RX-7 I can't vote in the poll, but I do use overrev fuel cut on my Microtech and bounce off it all the time without issues.

And premix would be related to this how?

 

It was the only possible scenario I could think of that might present a problem, no fuel, no oil injection (running only premix) maybe causing a seal to be damaged? Not that it is very likely, I could have phrased it better I suppose.

 

The chrome coating on the rotor housings are porous to trap the oil film.

 

True, and I suppose you really can't run it dry in that situation...I stand corrected.

 

It's funny how these forums have come full circle. People are more careful when it comes to removing factory systems (fuel cut, idle related stuff). They think it through carefully and weigh the pros and cons, instead of just ditching things because they are stock.

I for one have detonated the hell out of my motor on stock turbo due to overboost. The ported wastegate still couldn't flow enough with ambient temps of 35 F or so, boosting in 4th. This was an s5 turbo. The car hit 18psi. All I heard was thunk and I just felt a lump in my throat. Good thing the motor survived. I just wish I could have raised the fuel cut to something like 14psi instead.

With my Power FC I can set my overboost fuel cut. I have it configured for 17psi right now.

 

yeah that whole "i dont know what it is so i took it off and now may car doesnt run right" thing got old! and b) where i live its crowded and need to pass smog, so 146hp in a 2800lbs car is about twice as you need.

my friends FD has some issue with the pre control, on the 1st turbo it hits fuel cut, or about 18psi, whichever, and its been this way for 2 years

 

wow

 

So, the fuel cut is only cutting fuel from being delivered to the rear rotor?

arghx, that's quite the spike, haha, I'd be scared shitless.

What a contribution to the thread.

 

Yes, fuel is cut to the rear rotor only on the stock ECU. The decrease in exhaust flow lets the turbo slow down and stop making too much boost.

 

On my old engine I hit fuel cut over 8 times and never had any issues directly afterward. It lost compression from the apex seal springs flattening out, also the fuel pump had 5V going to it. It wouldn't start after a 4th gear 120mph drive (with time to cool down). I'm not sure if fuel cut contributed to it but I did hear clunks a few times when it would hit fuel cut which could've been detonation.

 

Wait, what is the factory fuel cut at? I don't remember what psi it is. I finally have a N370 ecu with my 6 port set up, but I'm hitting 10 psi. No fuel cuts, or fbcd.

 

I have seen and heard of many motors blowing up here in Portland, however it has not been a link to the stock over boost fuel cut. Most of the time is has been because the cars have been modified and have been tuned incorrectly. Timing was wrong, or the cars fuel mixtures have been set up wrong. Just last week a guys motor blew up because a company set the firing order of the leading and trailing wrong. They had the trailing fire first and then the leading. A friend of mine has also blown up there motor because the boost spiked and the car was too lean and it popped the motor. These are mainly 3rd gens though.

 

Fuel cuts at 8 psi. Maybe its been RTEK'd?